1. Tied hashes between two systems (e.g. my hosted website and my local mirror) can be incompatible, depending on the db engines employed.
2. Tied hashes normally encompass only one level. Hashes containing arrays and other hashes can be cumbersome to implement.

What I was hoping to achieve was a portable, readable file capable of containing a complex Perl data structure, that I could slurp into memory, manipulate, and write back out.

XML::Simple is the putative answer to these goals. It was pretty simple, but not brain-dead simple. Some of the arrays embedded in my hash have only one element. Apparently XML, by itself, is unable to distinguish between a single-element array and a scalar, so there is no a priori one-to-one correspondence between plain XML and a Perl data structure. XML::Simple gives you a way to force certain elements to be arrays when the XML file is read, but this amount of finagling was contrary to my objectives.

Would I use XML again? Probably not as a general-purpose embodiment of a Perl data structure. For that, I would look around for another format, another module -- or write my own. But my mind is still open for other applications. With this much smoke and heat, there's gotta be a fire somewhere!